1. Field of the Invention
This invention relates to printing methods and systems of the type in which discrete areas or pixels are varied in intensity to create an image on a substrate. More specifically, the invention relates to a system or method for intensity-variable printing which provides smoother image quality with less memory use than systems heretofore known.
2. Description of the Prior Art
Intensity-variable printing is, for the purposes of this document, defined as any type of printing in which colorant may be deposited on a base unit area, or pixel, in more than one intensity or degree of dilution. Diffusion printing, which is defined as including both common dye-diffusion and sublimation dye printing techniques, is one type of intensity-variable printing. Other intensity-variable printing techniques may include continuous ink jet printing, variable spot-sized phase change ink jet printing, variable dot electrophotography, variable dot thermal transfer printing, laser-induced dye diffusion transfer processes, dry silver technology and photographic printing, as well as others.
Dye diffusion printing involves the transfer of a dye colorant from a carrier, such as a polymer ribbon, onto a specialized substrate surface, such as a polyester sheet or a coated sheet of ordinary paper, in a controlled manner to generate an image. A thermal transfer print engine having an array of electrically actuatable heating elements is typically positioned so that the ribbon is juxtaposed between the print head elements and the substrate. The amount or intensity of dye deposited at a single location or pixel on the substrate can be varied by adjusting the electrical input to the print head elements. When one or more of the print head elements is heated, the dye/carrier structure is heated to a temperature sufficient to cause migration of a commensurate amount of dye to the substrate. By using a ribbon or ribbons having the three primary or subtractive colors, or those colors plus black, highly subtle colorant gradations may be achieved on the substrate. Images of near photo quality have been produced using dye diffusion technology.
One disadvantage that is present in some such systems is the tendency to produce unwanted patterning and printer-created artifacts. For example, patterning might result if more than one of the possible inputted intensity-specifying values is assigned to a single power input level for the heating elements of the print head, as would be the case if a large range of intensity-specifying values were to be compressed over a smaller range of selected power input levels.
In designing such systems, it is also desirable to keep printer memory requirements to a minimum. Memory adds to the cost of the printer, and printer speed and performance will tend to decrease as the degree of memory storage and manipulation escalates.
It is clear there has existed a long and unfilled need in the prior art for an intensity-variable printing system and method which abates patterning and printer-created artifacts, which provides smoother shading of colors, and which uses less memory space than systems and methods heretofore known.